Geladas, found only in the remote mountains of Ethiopia, use an undulating rhythm of lip-smacks in friendly encounters, featuring rapid fluctuations in pitch and volume not found in the typically one- or two-syllable calls of other monkeys and apes.
Writing in the journal Current Biology, the researchers suggest the finding is evidence lip-smacking could have been an evolutionary step toward human speech.
"Our finding provides support for the lip-smacking origins of speech because it shows that this evolutionary pathway is at least plausible," said Thore Bergman of the University of Michigan. "It demonstrates that nonhuman primates can vocalize while lip-smacking to produce speech-like sounds."
Bergman said he was struck by the distinctive sounds gelada made when he began his fieldwork in 2006.
"I would find myself frequently looking over my shoulder to see who was talking to me, but it was just the geladas," he said. "It was unnerving to have primate vocalizations sound so much like human voices."
Analysis of recordings of the geladas' vocalizations, known as "wobbles," has detected a rhythm corresponding to the opening and closing of parts of the mouth found in both lip-smacking and human speech, he said.
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